Filter device for polycarbonate and production method for polycarbonate

ABSTRACT

Provided are a filtration apparatus for a polycarbonate comprising a filtration container, a flange plate, one or more leaf disc-type filters, a filter press and a center pole having an integral construction with the filter press as constituting elements, these components having specific structures, and a process for manufacturing a polycarbonate using the filtration apparatus. Foreign materials are effectively removed by using the polymer filter, and at the same time, discoloring, crosslinking and gel formation, which would occur in the filter, are suppressed; and as a result, a polycarbonate having excellent quality can be manufactured.

TECHNICAL FIELD

The present invention relates to a process for manufacturing apolycarbonate having few foreign materials. More specifically, thisinvention relates to a process for manufacturing a polycarbonate havingfew foreign materials by using a polymer filter.

BACKGROUND ART

Generally, foreign materials in polycarbonates are classified intoforeign materials got mixed from raw materials or from the outside of areaction system, and those generated in a reaction apparatus or in apassage for a highly viscous material obtained by reaction. Against theformer, a preventing means for contamination comprises the use of afilter for filtering foreign materials in raw materials or theimprovement of closeness (tightness) of the reaction system. On theother hand, against the latter, foreign materials are removed by using afiltration filter directly before the highly viscous material isprocessed into a desired form.

DISCLOSURE OF THE INVENTION

However, depending on the temperature or viscosity of a melting polymerto be filtered, the retained particle diameter of a filtration filter orthe amount of treated materials, deterioration attributable to retentionsuch as discoloring, cross-linking, gel generation would occur in afiltration filter, exerting great influence on the product quality.

Recently, especially in a polycarbonate used for optical applicationssuch as DVD, MO or CD-R, which require high density and high accuracy,problems of foreign materials, discoloring, and gel generation havedirect influence on optical properties such as block error rate, or onmechanical properties such as tensile strength, flexural strength andtoughness of the final products, and these problems therefore areserious. Further, because gels characteristically change their shapes,those having a size even larger than the retained particle diameter of afilter would pass the filter, in some case, and this causes extremelysevere problems.

The object of the present invention is to provide a process for solvingproblems of the above-mentioned prior arts, efficiently removing foreignmaterials by using a polymer filter, and suppressing the occurrence ofdiscoloring, crosslinking and gel formation in the filter, at the sametime, in order to manufacture a polycarbonate having excellent quality.

The present invention comprises the following.

1. A filtration apparatus for a polycarbonate comprising as constituentelements:

(a) a filtration container having a polymer inlet passage and anopening;

(b) a flange plate interfitting with the opening;

(c) one or more leaf disc-type filters for filtering a polymerintroduced into the filtration container through the polymer inletpassage;

(d) a filter presser for fixing the leaf disc-type filters to each otherand applying sealing pressure on the junction parts of the leafdisc-type filters; and

(e) a center pole placed on a space formed by the inner circumferencesof the leaf disc-type filters, for collecting the polymer filtered bythe leaf disc-type filters and for discharging it outside the filtrationapparatus, and integrated with the filter presser.

2. A filtration apparatus according to above 1 characterized in that thecenter pole penetrates the flange plate, the flange plate has a sealingpart for preventing the intrusion of a polymer down along the centerpole into the penetration part of the center pole on the filtrationcontainer side of the flange plate, and the flange plate has a fixturefor fixing the center pole with the flange plate on the side opposite tothe filteration container of the flange plate.

3. A filtration apparatus according to above 1 or 2 characterized inthat the center pole has a polymer flow passage through the inner sidethereof on the portion corresponding to the section where the centerpole penetrates the flange plate.

4. A filtration apparatus according to any one of above 1 to 3characterized in that the shape of the cross section of the center poleis substantially a polygon or star shape having plural apexes and thelength of each portion where the center pole contact with the innercircumference of the leaf disc-type filter is 3 mm or less on the crosssection.

5. A filtration apparatus according to any one of above 1 to 4characterized in that cross-sectional area of polymer flow passage whichform between the center pole and the inner circumferences of a leafdisc-type filter increases continuously as it goes downstream along thepolymer flow.

6. A filtration apparatus according to any one of above 1 to 5characterized in that the boundary formed between the surface (a)substantially perpendicular to the center pole which is on thefiltration container side of the flange plate, and the surface otherthan this surface (a) is a curved surface having a curvature of 1 mm ormore.

7. A filtration apparatus according to any one of above 1 to 6characterized in that the polycarbonate has a viscosity averagemolecular weight of 10,000 or more.

8. A filtration apparatus according to any one of above 1 to 7characterized in that the polycarbonate is manufactured by polymerizingan aromatic diol compound and a carbonic acid diester compound in thepresence or absence of a catalyst.

9. A process for manufacturing a polycarbonate characterized in that afiltration apparatus according to any one of above 1 to 8 is used forfiltering the polymer.

It was made clear that, being different from polyesters such aspolyethylene terephthalate, when a polycarbonate is heated for a longtime even in non-oxygen atmosphere at such a high temperature as isenough to melt a polycarbonate, it forms a crosslinked structure, andeventually a material, called gel, having a viscoelastic behaviordifferent from that of polycarbonate, and an insoluble substance, andthe hue is also deteriorated.

For this reason, it is important to reduce, as much as possible,sections where polymer stagnates, which is called dead space, in thefiltration of a polycarbonate.

Filtration apparatuses conventionally used for filtering a polymerinclude those of a candle type and a plate type besides a leaf disctype, but the leaf disc-type filtration apparatus is preferred forfiltering a polycarbonate from the view points of the above-mentioneddead space, filtration area and the exchangeability of filters.

However, in the leaf disc type filtration apparatus, because pluraltoroidal filters to be used are stacked up so that the polymer filteredthrough them is collected and discharged outside the filtrationapparatus via a center pole (a hollow or grooved rod), sealing membersplaced on the inner circumference of the toroidal filters must beclosely joined to each other so that the unfiltered polymer existingoutside the filters does not mix with the filtered polymer existinginside the filters or the center pole. Hence, in a conventional leafdisc-type filtration apparatus, a filter presser is placed at an end ofthe stacked leaf disc-type filters, a center pole is placed through aspace formed by the inner circumference of the stacked leaf discfilters, and these elements are tightened with fixtures such as boltsand nuts in order to assemble the filtration apparatus.

Accordingly, the bolts and nuts used for tightening come to contact withthe molten polymer, and gaps formed at tightening parts act as deadspace, resulting with problems such as deterioration of the hue of theobtained polymer and gel formation.

According to the present invention, the filter presser and the centerpole have an integral structure, whereby dead space is not formed, andbolts for tightening the center pole and the filters are not needed.Further, when nuts are made not to contact with a molten polymer, deadspace is not formed, and the deterioration of polymer is prevented tomake possible to produce a polycarbonate having excellent quality. Here,“the filter presser and the center pole have an integral structure”means that the filter presser and the center pole are not separable intoplural parts.

In the present invention, it is preferable that the shape of crosssection of the center pole in the direction perpendicular to its axiswhere the center pole contacts with the inner circumference of a leafdisc-type filter is substantially a polygon or a star shape havingplural apexes. Such a shape allows that the filtered polymer flows outtoward the outside of the system through a polymer passage formed by theouter circumference of the center pole (that is, the circumferenceformed by linking the apexes of the external polygonal or star-likeshape in the cross-section of the center pole part in the directionperpendicular to its axis), the grooves carved on the center pole part,and the inner circumferences of the leaf disc-type filters, andtherefore, dead space formed to a considerably lesser extent than in thecase of a hollow center pole. Further, the area where the innercircumferences of the leaf disc-type filters are in contact with theouter circumference of the center pole is small. Thus, the dead spacegeneration on the contacting surfaces can be avoided. In this regard,the length of each portion where the center pole contact with the innercircumference of the leaf disc-type filter is more preferably 3 mm orless on the cross section.

Further, while the amount of the filtered polymer flowing through thepolymer passage of the center pole increases as the flow approaches theflange plate, that is, the amount increases as the flow of the polymergoes downstream, it is preferable that the pressure loss caused by theflow of the polymer is made small in order to realize good filtration.For this purpose, the cross sectional area of polymer flow passage whichform between the center pole and the inner circumferences of a leafdisc-type filter preferably increases continuously as the polymer flowgoes downstream along the polymer flow. In concrete, for example, thedepth of the grooves formed on the center pole is preferably made deeperas the polymer flow approaches the flange plate.

In the present invention, the center pole preferably has a hollowpolymer passage in the area where it penetrates the flange plate. Bythis structure, it becomes possible that fixtures such as a fixture nutfor the center pole are prevented from contact with polymer.

In the present invention, when a polymer flows inside of the filtrationapparatus formed by the filtration container and the flange plate, thepolymer flow direction is changed drastically near the juncture of thefiltration container and the flange plate, and dead space is apt to begenerated there. So, a corner having a curved surface of a curvature of1 mm or more is preferably formed on the flange plate side.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of the filtration apparatus of the presentinvention.

FIG. 2 is the A—A cross-sectional view of FIG. 1.

FIG. 3 shows a prior art filtration apparatus.

In FIGS. 1 and 2 the polymer before filtration is introduced into theinside of a filtration apparatus formed with a filtration container 1and a flange plate 2 via a polymer flow passage 3 formed in thefiltration container 1. Inside the filtration apparatus, one or moreleaf disc-type filters 5 and spacers 6 are stacked together, and acenter pole 7 is placed in the space formed by the inner circumferencesof the leaf disc-type filters. As a sealing member for preventing theintrusion of the polymer along the center. pole from entering into theabove-mentioned penetration part, a sealing packing 9 is placed betweena filter presser 4 for the leaf disc-type filter 5 and the flange plate2 on the filter container side of the flange plate. Here, the filterpresser 4 and the center pole 7 have an integral construction having noseam, and the center pole 7 penetrates the flange plate 2. By applying asealing pressure through tightening a fixture, that is a fixture nut 8,which is placed on the flange plate 2 opposite to the filtrationcontainer, the sealings of joining parts of the leaf disc-type filters,that is, the sealing between each of the leaf disc-type filters 5 with aspacer 6 in-between, the sealing between a leaf disc-type filter 5 andthe filter presser 4 with a spacer 6 in-between, and the sealing betweena leaf disc-type filter 5 and the flange plate 2 in-between with aspacer 6, are achieved, thereby fixing the center pole and the flangeplate.

The polymer introduced into the inside of the filtration apparatus isfiltered with the leaf disc-type filters 5, passes through the leafdisc-type filters 5, is collected into the polymer passage 10 formed bythe inner circumferences of the leaf disc-type filters 5, the outercircumference of the center pole 7 and the grooves carved on the surfaceof the center pole, and is subsequently discharged outside the systemvia a hollow polymer passage (a hole formed inside the center pole) 11in such a manner that the polymer passes inside the center pole at theportion where the center pole penetrates the flange plate.

Further the filter container 1 and the flange plate 2 are joined withoutgaps in metal touch, using a hollow metallic O-rings or the like on thejoining surfaces as required. The boundary 12 between the surfacesubstantially perpendicular to the center pole 7 which is also on thefiltration container side of the flange plate 2, and the other surface,is finished so that it has a curved surface having a curvature of 1 mmor more so as to prevent dead space generation.

Further, heaters are commonly placed outsides the filtration container 1and the flange plate 2, and a detecting element for controllingtemperature is placed on the filtration container 1, although these arenot shown in FIG. 1.

The material of the leaf disc-type filter 5 is not specifically limitedas far as it is inactive against a polycarbonate obtained bypolymerization and does not generate any eluate into the polycarbonate;however, a metal, especially stainless steel is commonly used; forexample, SUS304, SUS316 or the like is preferably used.

FIG. 3 shows a filtration apparatus partially lacking a part of theinvention specifying elements of the present invention to compare thepresent invention with prior arts.

In FIG. 3, a center pole 7 is not integrated with a filter presser 4,but it is integrated with a flange plate 2, and the filter presser 4 isfixed to leaf disc-type filters 5 by inserting a fixing bolt 13 into athread groove of the center pole 7 and tightening it. Further, theboundary 15 between the surface substantially perpendicular to thecenter pole which is also on the filtration container side of the flangeplate 2, and the other surface (the surface where the filtrationcontainer 1 is in contact with the flange plate 2), forms a right anglecross-section and is not curved.

The polymer introduced into the inside of the filtration apparatus isfiltered with the leaf disc-type filters 5, passes through the leafdisc-type filters 5 and is taken out outside the system via a hollowpolymer passage 14.

There is no specific limitation on the aromatic polycarbonate of thepresent invention, and an aromatic polycarbonate obtained by reacting anaromatic diol compound with a carbonate precursor is usable. Examples ofthe polycarbonate include an interfacial polymerization polycarbonate,which is obtained through the reaction of an alkali metal salt of anaromatic diol with phosgene and a melt polymerization polycarbonate,which is obtained through the reaction of an aromatic diol with anaromatic carbonic acid diester. Among these aromatic polycarbonates, thepolycarbonate which is obtained by melt polymerization is most suitedfor executing the present invention because it can be obtained directlyin a molten state from a polymerization reactor and does not needremelting of the polymer.

The method for manufacturing a polycarbonate through melt polymerizationmeans the method in which an aromatic diol compound and a carbonic acidester are subjected to melt polycondensation in the presence of atransesterification catalyst consisting of a basic nitrogen compound, analkali metal compound and/or an alkaline earth metal compound, or thelike.

Concrete examples of the aromatic diol compound includebis(4-hydroxyphenyl)methane, 2,2-bis(4-hydroxyphenyl)propane,2,2-bis(4-hydroxy-3-methylphenyl)propane,4,4-bis(4-hydroxyphenyl)heptane,2,2-bis(4-hydroxy-3,5-dichlorophenyl)propane,2,2-bis(4-hydroxy-3,5-dibromophenyl)propane, bis(4-hydroxyphenyl)oxide,bis(3,5-dichloro-4-hydroxyphenyl)oxide, p,p′-dihydroxydiphenyl,3,3′-dichloro-4.4′-dihydroxydiphenyl, bis(hydroxyphenyl)sulfone,resorcinol, hydroquinone, 1,4-dihydroxy-2,5-dichlorobenzene,1,4-dihydroxy-3-methylbenzene, bis(4-hydroxyphenyl)sulfidebis(4-hydroxyphenyl)sulfoxide, or the like. Out of these,2,2-bis(4-hydroxyphenyl)propane is especially preferable.

As the carbonic acid diester, concretely, diphenyl carbonate, ditolylcarbonate, bis(chlorophenyl) carbonate, m-cresyl carbonate, dinaphthylcarbonate, bis(diphenyl) carbonate, dimethyl carbonate, diethylcarbonate, dibutyl carbonate, dicyclohexyl carbonate or the like isused. Among these compounds, diphenyl carbonate is preferred.

Further, the polycarbonate in the present invention is allowed tocontain as necessary; for example, ethylene glycol, 1,4-butanediol,1,4-cyclohexanedimethanol, 1,10-decanediol or the like as an aliphaticdiol compound; also for example, succinic acid, isophthalic acid,2,6-naphthalenedicarboxylic acid, adipic acid,. cyclohexanedicarboxylicacid, terephthalic acid or the like as a dicarboxylic acid compound; andalso a hydroxycarboxylic acid compound, for example, lactic acid,p-hydroxybenzoic acid, 6-hydroxy-2-naphthoic acid or the like.

Examples of the alkali metal compound used as the catalyst includehydroxides, hydrogen carbonates, carbonates, acetic acid salts, nitricacid salts, nitrous acid salts, sulfurous acid salts, cyanic acid salts,thiocyanic acid salts, stearic acid salts, boron hydride salts, benzoicacid salts, hydrogen phosphoric acid salts, bisphenol salts and phenolicsalts, etc., of alkali metals.

Concrete examples include sodium hydroxide, potassium hydroxide, lithiumhydroxide, sodium hydrogen carbonate, potassium hydrogen carbonate,lithium hydrogen carbonate, sodium carbonate, potassium carbonate,lithium carbonate, sodium acetate, potassium acetate, lithium acetate,sodium nitrate, potassium nitrate, lithium nitrate, sodium nitrite,potassium nitrite, lithium nitrite, sodium sulfite, potassium sulfite,lithium sulfite, sodium cyanate, potassium cyanate, lithium cyanate,sodium thiocyanate, potassium thiocyanate, lithium thiocyanate, sodiumstearate, potassium stearate, lithium stearate, sodium borohydride,potassium borohydride, lithium borohydride, sodium phenyl borate, sodiumbenzoate, potassium benzoate, lithium benzoate, disodiumhydrogenphosphate, dipotassium hydrogenphosphate and dilithiumhydrogenphosphate; disodium salt, dipotassium salt and dilithium salt ofbisphenol A; sodium salt, potassium salt and lithium salt of phenol; andthe like.

Examples of the alkaline earth metal compound used as the catalystinclude hydroxides, hydrogencarbonates, carbonates, acetic acid salts,nitric acid salts, nitrous acid salts, sulfurous acid salts, cyanic acidsalts, thiocyanic acid salts, stearic acid salts, benzoic acid salts,bisphenol salts, phenolic salts, etc., of alkaline earth metals.

Concrete examples include calcium hydroxide, barium hydroxide, strontiumhydroxide, calcium hydrogencarbonate, barium hydrogencarbonate,strontium hydrogencarbonate, calcium carbonate, barium carbonate,strontium carbonate, calcium acetate, barium acetate, strontium acetate,calcium nitrate, barium nitrate, strontium nitrate, calcium nitrite,barium nitrite, strontium nitrite, calcium sulfite, barium sulfite,strontium sulfite, calcium cyanate, barium cyanate, strontium cyanate,calcium thiocyanate, barium thiocyanate, strontium thiocyanate, calciumstearate, barium stearate, strontium stearate, calcium borohydride,barium borohydride, strontium borohydride, calcium benzoate, bariumbenzoate and strontium benzoate; calcium salt, barium salt and strontiumsalt of bisphenol A; calcium salt, barium salt and strontium salt ofphenol; and the like.

In the present invention, as desired, (a) an alkali metal salt of an atecomplex of an element of the 14th group of the periodic table or (b) analkali metal salt of an oxo acid of an element of the 14th group of theperiodic table can be used as the alkali metal compound of the catalyst.The elements of the 14th group of the periodic table means silicon,germanium and tin. (a) The alkali metal salts of the ate complex of theelement of the 14th group of the periodic table are those described inJP-A 7-268091 (JP-A means Japanese unexamined patent publication).Concrete examples of the alkali metal salts include germanium (Ge)compounds such as NaGe(OMe)₅, NaGe(OEt)₃, NaGe(OPr)₅, NaGe(OBu)₅,NaGe(OPh)₅, LiGe(OMe)₅, LiGe(OBu)₅ and LiGe(OPh)₅.

Examples of the compound of tin (Sn) are NaSn(OMe)₃, NaSn(OMe)₂(OEt),NaSn(OPr)₃, NaSn(O-n-C₆H₁₃)₃, NaSn(OMe)₅, NaSn(OEt)₅, NaSn(OBu)₅,NaSn(O-n-C₁₂H₂₅)₅, NaSn(OEt), NaSn(OPh)₅, and NaSnBu₂(OMe)₃.

(b) Also, as the alkali metal salt of the oxo acid of the element of the14th group of the periodic table, for example, alkali metal salts ofsilicic acid, stannic acid, germanous (II) acid, and germanic (IV) acidare cited as preferable.

The alkali metal salt of silicic acid is, for example, an acidic orneutral alkali metal salt of monosilicic acid or condensed compoundthereof of which examples include monosodium orthosilicate, disodiumorthosilicate, trisodium orthosilicate and tetrasodium orthosilicate.

The alkali metal salt of stannic acid is, for example, an acidic orneutral alkali metal salt of monostannic acid or condensed compoundthereof of which examples include disodium monostannate (Na₂SnO₃.XH₂O,X=0 to 5) and tetrasodium monostannate (Na₄SnO₄).

The alkali metal salt of germanous (II) acid is, for example, an acidicor neutral alkali metal salt of monogermanous acid or condensed compoundthereof of which examples include monosodium germanate (NaHGeO₂).

The alkali metal salt of germanic (IV) acid is, for example, an acidicor neutral alkali metal salt of monogermanic (IV) acid or condensedcompound thereof of which examples include monolithium orthogermanate(LiH₃GeO₄), disodium orthogermanate, tetrasodium orthogermanate,disodium digermanate (Na₂Ge₂O₅), disodium tetragermanate (Na₂Ge₄O₉) anddisodium pentagermanate (Na₂Ge₅O₁₁)

An alkali metal compound or alkaline earth metal compound as thecatalyst is preferably used in such a manner that an alkali metalelement or alkaline earth metal element in the catalyst is 1×10⁻⁸ to5×10⁻⁵ equivalent based on 1 mol of an aromatic diol compound. Morepreferable ratio is attained by using 5×10⁻⁷ to 1×10⁻⁵ equivalent on thesame basis.

When the amount of an alkali metal element or alkaline earth metalelement in the catalyst deviates from the range of 1×10⁻⁸ to 5×10⁻⁵equivalent based on 1 mol of an aromatic diol compound, unfavorabletroubles are caused, that is, bad influence is exerted on severalproperties of the obtained aromatic polycarbonate, thetransesterification reaction does not sufficiently proceed, and as aresult, an aromatic polycarbonate having high molecular weight is notobtained, or the like.

Also, examples of the nitrogen-containing basic compound as the catalystinclude; ammonium hydroxides having an alkyl, aryl, alkylaryl group orthe like such as tetramethylammonium hydroxide (Me₄NOH),tetraethylammonium hydroxide (Et₄NOH), tetrabutylammonium hydroxide(Bu₄NOH), benzyltrimethylammonium hydroxide (F—CH₂(Me)₃NOH) andhexadecyltrimethylammonium hydroxide; tertiary amines such astriethylamine, tributylamine, dimethylbenzylamine andhexadecyldimethylamine; and basic salts such as tetramethylammoniumborohydride (Me₄NBH₄), tetrabutylammonium borohydride (Bu₄NBH₄),tetrabutylammonium tetraphenyl borate (Me₄NBPh₄) and tetrabutylammoniumtetraphenyl borate (Bu₄NBPh₄).

The above-mentioned nitrogen-containing basic compound is preferablyused in such a manner that the amount of the compound is 1×10⁵ to 5×10³,more preferably 2×10⁻⁵ to 5×10⁻⁴, especially preferably 5×10⁻⁵ to 5×10⁴equivalent in terms of ammoniacal nitrogen atom in thenitrogen-containing basic compound based on 1 mol of the aromatic diolcompound.

In this description of the present invention, the ratio of the alkalimetal compound, alkaline earth metal compound or nitrogen-containingbasic compound to the loaded aromatic diol compound (also referred to asan aromatic dihydroxy compound) has been expressed as “the amount of Z(compound name) in W (numerical value) equivalent in terms of a metal orbasic nitrogen based on one mole of an aromatic dihydroxy compound”.This means that, for example, when Z has one sodium atom like in sodiumphenoxide or 2,2-bis(4-hydroxyphenyl)propane monosodium salt, or Z hasone basic nitrogen like in triethylamine, the amount of Z is the amountcorresponding to W mol, and when Z has two sodium atoms like in2,2-bis(4-hydroxyphenyl)propane disodium, the amount of Z is the amountcorresponding to W/2 mol.

In polycondensation reaction of the present invention, at least one kindof a co-catalyst selected from a group consisting of an oxo acid and anoxide of an element of the 14th group of the periodic table is allowedto coexist with the above catalyst as necessary.

By using such a co-catalyst in a specific ratio, it is possible tosuppress more effectively unfavorable side reactions such as thebranching reaction, which tends to occur in the polycondensationreaction, the generation of foreign materials in an apparatus during themolding process, and yellowish, without giving adverse effects on theterminal blocking reaction and the reaction velocity ofpolycondensation.

There is no particular limitation to molecular weight of thepolycarbonate used in the present invention, however, the polycarbonatehaving a lower molecular weight has poor properties, and its usage isextremely limited, so that a polycarbonate having a viscosity averagemolecular weight of 10,000 or more is preferred. On the other hand, inthe case of having an extremely high molecular weight, an operatingpressure for filtration by a polymer filter increases, whereby aviscosity average molecular weight of 50,000 or less is preferable forexecuting the present invention.

In the present invention, the polycarbonate formed throughpolymerization may be filtered directly, or it may be filtered after thepolymerization catalyst contained in the polymer has been deactivated.

As a catalyst deactivator to be used for deactivating the polymerizationcatalyst, known catalyst deactivators are effective, and among them,ammonium salts and phosphonium salts of sulfonic acid are preferred; andammonium salts and phosphonium salts of dodecylbenzenesulfonic acid suchas dodecylbenzenesulfonic acid tetrabutylphosphonium salt anddodecylbenzenesulfonic acid tetrabutylammonium salt, and ammonium saltsand phosphonium salts of p-toluenesulfonic acid such asp-toluenesulfonic acid tetrabutylphosphonium salt or p-toluenesulfonicacid tetrabutylammonium salt are especially preferred.

Further, an ester of sulfonic acid is also a preferable catalystdeactivator. As esters of sulfonic acids, methyl benzenesulfonate, ethylbenzenesulfonate, butyl benzenesulfonate, octyl benzenesulfonate, phenylbenzenesulfonate, methyl p-toluenesulfonate, ethyl p-toluenesulfonate,butyl p-toluenesulfonate, octyl p-toluenesulfonate, phenylp-toluenesulfonate and the like are preferred.

Among these compounds, dodecylbenzenesulfonic acid tetrabutylphosphoniumsalt is the most preferred.

The quantity of use of such a catalyst deactivator is in a range of 0.5to 50 moles, preferably 0.5 to 10 moles, more preferably 0.8 to 5 molesbased on one mole of an above-mentioned polymerization catalyst selectedfrom alkali metal compounds and/or alkaline earth metal compounds.

Such a catalyst deactivator or other additives are added directly or asa solution or a dispersion using an appropriate solvent or polymer, oras a master pellet to a molten polycarbonate, and the mixture iskneaded. Although there is no particular limitation to an apparatus usedfor performing these operations, for example, a twin screw extruder orthe like is preferred, and in the case where a catalyst deactivator oranother additive has been dissolved or dispersed in a solvent, a twinscrew extruder having a vent is especially preferred.

Further, each reference numeral in the figures has the followingmeaning.

1. Filtration container.

2. Flange plate.

3. Polymer inlet passage.

4. Filter presser.

5. Leaf disc-type filter.

6. Spacer.

7. Center-pole.

8. Fixture nut.

9. Sealing packing.

10. Polymer flow passage.

11. Hollow polymer passage.

12. Boundary having a curved surface.

13. Fixture bolt.

14. Hollow flow passage.

15. Boundary having no curved surface.

EXAMPLES

Examples of the present invention are shown hereinafter. The Examplesare for exemplifying the present invention, and the present invention isnot restricted by them.

Example 1

A polycarbonate was filtered after it had been molten and kneaded with atwin screw extruder, by using a filtration apparatus as shown in FIG. 1,wherein the filtration apparatus comprising a filtration containerhaving a polymer inlet passage and an open part, as well as a flangeplate engaging with the opening part, and wherein a filter presser and acenter pole was integrated; the cross section of the center pole wassubstantially triangular; the contact length of the portions where theouter circumference of the center pole was in contact with the innercircumference of a leaf disc-type filters was 1 mm; the cross-sectionalarea of a polymer passage continuously changed from the upstream side tothe downstream side of the polymer flow so that the cross-sectional areaof the polymer passage on the most downstream side of the center pole(regarding the polymer flow in the center pole, the cross-sectional areaof the polymer passage at the position of the leaf disc-type filterexisting at the most distant position from the polymer inlet passage 3)was 1.5 times the cross-sectional area of the polymer passage on themost upstream side (regarding the polymer flow in the center pole, thecross-sectional area of the polymer passage at the position of the leafdisc-type filter existing at the nearest position to the polymer inletpassage 3); and further the boundary (corner) between the surfacesubstantially perpendicular to the center pole which was also on thefiltration container side of the flange plate, and the other surface,had a curvature of 3 mm. The aperture of the leaf disc-type filters was20 μm and the filtration area was 0.3 m².

At a polycarbonate filtration rate of 400 kg/hr per 1 m², 60 tons of thepolycarbonate in total was filtered at 270° C.

After the filtration was completed, the remained polycarbonate wasremoved with a solvent, and then, the inside of the filtration apparatuswas inspected under white light and ultraviolet-fluorescent lamps, andno remained substances were detected.

The polycarbonate used in the example was a polymer having a viscosityaverage molecular weight of 15,200 obtained from bisphenol A anddiphenyl carbonate used as raw materials through transesterification inthe presence of bisphenol A disodium salt and tetramethyl ammoniumhydroxide as catalysts.

Comparative Example 1

A polycarbonate was filtered after it had been molten and kneaded with atwin screw extruder, by using a filtration apparatus having thestructure as shown in FIG. 3, wherein a filter presser and a center polewere separated from each other, a filter presser was fixed to the centerpole with a fixing bolt, the external cross-sectional shape of thecenter pole was hexagonal, the center pole had such a structure that thefiltered polymer flowed through a polymer passage formed by the outercircumference of the center pole, the grooves carved on the center polepart and the inner circumferences of the leaf disc-type filters, andentered, as appropriate, into plural hollow passages formed in the flowdirection, the contact length of the portions where the outercircumference of the center pole was in contact with the innercircumferences of the leaf disc-type filters was 4 mm, thecross-sectional area of the polymer passage of the center pole was notchanged from the most upstream side to the most downstream side, andfurther, the surface substantially perpendicular to the center polewhich was also on the filtration container side of the flange plate, andthe other surface, were at right angles to each other at the boundary(corner) in-between and the boundary had no curved surface. The apertureof the leaf disc-type filters was 20 μm and the filtration area was 0.3m².

At a polycarbonate filtration rate of 400 kg/hr per 1 m², 60 tons of thepolycarbonate in total was filtered at 270° C.

After the filtration was completed, the remained polycarbonate wasremoved with a solvent, and then, the inside of the filtration containerwas inspected under white light and ultraviolet-fluorescent lamps.Adhered residual substances which had been discolored in brown weredetected at a fixing bolt, at the apexes of the hexagonal center pole,at the communication holes connecting the polymer passage formed by theouter circumference of the center pole, the grooves carved on the centerpole part and the inner circumferences of the leaf disc-type filters, tothe hollow passage of the center pole, and at the boundary between thesurface substantially perpendicular to the center pole which was also onthe filtration container side of the flange plate, and the othersurface.

The residual substances had luminescence from white to orange under anultraviolet-fluorescent lamp, and it was shown that the substances weregels formed by the retention of polymer.

The polycarbonate used in this example was a polymer having a viscosityaverage molecular weight of 15,200 obtained from bisphenol A anddiphenyl carbonate used as raw materials through transesterifrcation, inthe presence of bisphenol A disodium salt and tetramethyl ammoniumhydroxide as catalyst.

Industrial Field of Application

The present invention can solve the above-mentioned problems accompaniedby prior arts, efficiently remove foreign materials by using a polymerfilter, and at the same time, suppress discoloring, crosslinking and gelformation which occur in the filter; and as a result, it can produce apolycarbonate having excellent quality.

What is claimed is:
 1. A filtration apparatus for a polycarbonatecomprising as constituent elements: (a) a filtration container having apolymer inlet passage and an opening; (b) a flange plate interfittingwith the opening; (c) one or more leaf disc-shaped filters for filteringa polymer introduced into the filtration container through the polymerinlet passage; (d) a filter presser for fixing the disc-shaped filtersto each other and applying sealing pressure on junction parts of theleaf disc-shaped filters; and (e) a center pole placed on a space formedby inner circumferences of the leaf disc-shaped filters, for collectingthe polymer filtered by the leaf disc-shaped filters and for dischargingit outside the filtration apparatus, and integrated with the filterpresser, wherein the center pole includes a penetration part thatpenetrates the flange plate, the flange plate has a sealing part forpreventing the intrusion of a polymer down along the center pole intothe penetration part of the center pole on a filtration container sideof the flange plate, and the flange plate has a fixture nut for fixingthe center pole with the flange plate on a side opposite to thefiltration container of the flange plate.
 2. A filtration apparatusaccording to claim 1 characterized in that the center pole has a polymerflow passage through an inner side thereof on a portion corresponding toa section where the center pole penetrates the flange plate.
 3. Afiltration apparatus according to claim 1 characterized in that a shapeof a cross section of the center pole is substantially a polygon shapehaving plural apexes and a length of each portion where the center poleis in contact with the inner circumference of the leaf disc-shapedfilter is 3 mm or less on the cross section.
 4. A filtration apparatusaccording to claim 1 characterized in that a cross-sectional area of apolymer flow passage which forms between the center pole and the innercircumferences of said filters increases continuously as it goesdownstream along the polymer flow.
 5. A filtration apparatus accordingto claim 1 characterized in that a boundary formed between a surface (a)substantially perpendicular to the center pole which is on a filtrationcontainer side of the flange plate, and a surface other than thissurface (a) is a curved surface having a curvature of 1 mm or more.
 6. Afiltration apparatus according to claim 1, further comprisingpolycarbonate having a viscosity average molecular weight of 10,000 ormore.
 7. A filtration apparatus according to claim 6 characterized inthat the polycarbonate is manufactured by polymerizing an aromatic diolcompound and a carbonic acid diester compound with or without a catalystpresent.
 8. A process for filtering polycarbonate comprising the stepsof: providing a filtration apparatus according to any one of claims 1 to5; passing polycarbonate into said filtration container through saidpolymer inlet passage; passing polycarbonate through said one or moreleaf disc-shaped filters; collecting filtered polycarbonate from saidfilters with said center pole; and, discharging filtered polycarbonateto the outside of said filtration apparatus with said center pole.